Formation, spectroscopic characterization, and solution stability of an [Fe4S4]2+ cluster derived from ß-cyclodextrin dithiolate.

The formation and solution properties, including stability in mixed aqueous-Me(2)SO media, have been investigated for an [Fe(4)S(4)](2+) cluster derived from ß-cyclodextrin (CD) dithiolate. Clusters of the type [Fe(4)S(4)(SAr)(4)](2-) (Ar = Ph, C(6)H(4)-3-F) are generated in Me(2)SO by redox reactions of [Fe(4)S(4)(SEt)(4)](2-) with 2 equiv of ArSSAr. An analogous reaction with the intramolecular disulfide of 6(A),6(D)-(3-NHCOC(6)H(4)-1-SH)(2)-6(A),6(D)-dideoxy-ß-cyclodextrin (14), whose synthesis is described, affords a completely substituted cluster formulated as [Fe(4)S(4){ß-CD-(1,3-NHCOC(6)H(4)S)(2)}(2)](2-) (15). Ligand binding is indicated by a circular dichroism spectrum and also by UV-visible and isotropically shifted (1)H NMR spectra and redox behavior convincingly similar to [Fe(4)S(4)(SPh)(4)](2-). One formulation of 15 is a single cluster to which two dithiolates are bound, each in bidentate coordination. With there being no proven precedent for this binding mode, we show that the cluster [Fe(4)S(4)(S(2)-m-xyl)(2)](2-) is a single cubane whose m-xylyldithiolate ligands are bound in a bidentate arrangement. This same structure type was proposed for a cluster formulated as [Fe(4)S(4){ß-CD-(1,3-SC(6)H(4)S)(2)}(2)](2-) (16; Kuroda et al. J. Am. Chem. Soc.1988, 110, 4049-4050) and reported to be water-stable. Clusters 15 and 16 are derived from similar ligands differing only in the spacer group between the thiolate binding site and the CD platform. In our search for clusters stable in aqueous or organic-aqueous mixed solvents that are potential candidates for the reconstitution of scaffold proteins implicated in cluster biogenesis, 15 is the most stable cluster that we have thus far encountered under anaerobic conditions in the absence of added ligand.

Cubane-type Fe4S4 clusters with chiral thiolate ligation: formation by ligand substitution, detection of intermediates by 1H NMR, and solid state structures including spontaneous resolution upon crystallization.

Cubane-type clusters [Fe(4)S(4)(SR*)(4)](2-) containing chiral thiolate ligands with R* = CH(Me)Ph (1), CH(2)CH(Me)Et (2), and CH(2)CH(OH)CH(2)OH (3) have been prepared by ligand substitution in the reaction systems [Fe(4)S(4)(SEt)(4)]/R*SH (1-3, acetonitrile) and [Fe(4)S(4)Cl(4)](2-)/NaSR*(3, Me(2)SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe(4)S(4)L(4-n)(SR*)(n)](2-) (L = SEt, Cl) with n = 1-4. Clusters 1 and 2 were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (n = 1) and seven diastereomers and their enantiomers (n = 2-4). Reactions were monitored by isotropically shifted (1)H NMR spectra in acetonitrile or Me(2)SO. In systems affording 1 and 2 as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et(4)N)(2)[1] afforded 1-[SS(RS)(RS)] in which two sites are disordered because of occupancy of R and S ligands. Similarly, (Et(4)N)(2)[2] led to 2-[SSSS], a consequence of spontaneous resolution upon crystallization. The clusters 3-[RRRR] and 3-[SSSS] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with n = 1-4 by appearance of four pairs of diastereotopic SCH(2) signals in both acetonitrile and Me(2)SO reaction systems. Identical spectra were obtained with racemic, R-(-), and S-(+) thiols, indicating that ligand-ligand interactions are too weak to allow detection of diastereomers (e.g., [SSSS] vs [SSRR]). The stability of 3 in Me(2)SO/H(2)O media is described.